My radiator cooling fans only come on when 1) the AC is turned on and 2) the CTS (top or bottom) is unplugged. So far, I have switched the thermostat CTS and nothing changed. Relays are good, fuses are good, Ordered a new CTS for the radiator coolant temp sensor and also a new thermostat. Is there anything else worth checking prior to replacing these parts? Car is overheating when stopped due to the fans not kicking on when needed to.

 

What year, model, trim level is this?

 

2006 Honda Civic EX 1.8

 

There are 4 separate relays that are part of the cooling fan circuits. You'll need a multi-meter to do some tests.

Here is one test:
Remove the fan control relay (#4 in diagram) from the under-hood fuse/relay box.

Set the multi-meter to check for voltage.
Turn the ignition switch to the ON position. (Car not started or running.)
Touch the multi-meter red lead to terminal no. 5 of the fan relay control socket in the under-hood fuse/relay box. Multi-meter black lead touching body ground (bar metal on car chassis, ground bolt, etc.).

Is there battery voltage? Standard battery voltage is 12 volts.

If there is no voltage and under-dash Fuse No. 36 (10A) is good, then there is an open (break) in the circuit between those two points.